A magnetic device includes a magnetic core coupled to conductor windings such that the magnetic flux flows in a closed path about the core. Magnetic devices are generally configured in an EE-type arrangement or a toroidal geometry. In the EE-type magnetic device, a first and second core-portion of the magnetic core surround the conductor windings. In the toroidal magnetic device, a first and second winding-portion of the conductor windings surround the magnetic core.
Micromagnetic devices (e.g., microinductors or microtransformers) are micron-scaled integrated circuit magnetic devices; the electromagnetic properties of the device are provided by the presence of the magnetic core and conductor windings. In the past, micromagnetic devices were only applicable to low-level signal applications (e.g., recording heads). With the advancement in the production techniques for integrated circuits, it is now possible to fabricate micromagnetic devices for relatively large signal and power processing applications. For instance, micromagnetic devices may be employed in power systems for wireless communications equipment.
While many power semiconductor devices (having ferrite cores, for instance) have been scaled down into integrated circuits, inductive elements at the present time remain discrete and physically large of course, there is a strong desire to miniaturize these inductive components as well. By extending thin-film processing techniques employed in power semiconductor devices to ferromagnetic materials, the size of the conventional discrete ferromagnetic-core inductive devices can be reduced significantly. Ferromagnetic materials such as alloys, however, have much higher saturation flux densities than ferrites (e.g., 10–20 kG verses 3 kG) thereby reducing the physical volume of the core for a given inductance and energy requirement. To limit the eddy current losses in the ferromagnetic materials, the materials must be fabricated in inordinately thin films. Processing thin-film ferromagnetic materials with traditional rolling and tape winding techniques proves to be very costly as the desired tape thicknesses drops below 0.001 inches (i.e., 25 μm). It is thus advantageous to produce such thin films by other integrated circuit deposition techniques such as sputtering or electroplating.
Another germane consideration associated with manufacturing micromagnetic devices is securing the ferromagnetic material to a silicon substrate or the like. More specifically, forming an adequate bond between the ferromagnetic material and an insulator coupled to the substrate is an important consideration. Many factors such as oxide formation, melting point temperature, interposed contamination, affinity between materials and mechanical stress at the interface, influence the adhesion of a thin film to a substrate. For instance, one technique readily employed in thin film manufacturing processes is the formation of an oxide-metal bond at the interface between the substrate and the film. The oxide-metal bond may be formed by employing an oxygen-active metal (such as tungsten or chromium) on an oxygen-bearing substrate (such as glass or ceramic) in conjunction with a refractory metal (such as tantalum or tungsten). With regard to contaminants, it is advantageous to remove any impurities interposed on the substrate. Cleaning methods vary in effectiveness and the selected method required depends on the ability of the deposition process to dislodge contaminant atoms. As an example, different cleaning techniques may be employed with sputtering or electroplating.
Of course, the ultimate consideration with regard to the adhesion properties depends on the materials employed. While others have attempted to address the adhesion of ferromagnetic materials to an insulator coupled to a substrate [e.g., Measured Performance of a High-Power-Density Microfabricated Transformer in a DC-DC Converter, by Charles R. Sullivan and Seth R. Sanders, IEEE Power Electronics Specialists Conference, p. 287–294 (July 1996), which is incorporated herein by reference], to date, the problem remains unresolved. Developing an adhesive material that simultaneously forms a bond with the insulator and the ferromagnetic material such that thin-film processing can be applied to inductive elements will provide a foundation for the introduction of power micromagnetic integrated circuits.
Accordingly, what is needed in the art is an adhesive for a micromagnetic device that can secure a ferromagnetic core to an insulator coupled to a substrate such that the micromagnetic device can handle the more stringent requirements associated with power processing applications.